This invention relates to a method of controlling an inverter power generation apparatus having an inverter circuit to convert an output of an AC generator into an AC output of arbitrary frequency when it is operated in parallel to other power supply.
An inverter power generation apparatus has been used for a power generation apparatus driven by an internal combustion engine as a primer.
In general, the inverter power generation apparatus comprises an AC generator driven by the internal combustion engine, a DC power supply section to convert an output voltage of the AC generator into a DC voltage, an inverter circuit to convert an output voltage of the DC power supply section into an AC output of predetermined frequency, a filter circuit to remove a harmonic component from an output of the inverter circuit, load connection terminals having an output of the filter circuit applied and a controller having PWM control means to control the inverter circuit in the form of pulse width modulation (referred to as PWM later) so as to output the AC voltage of predetermined waveform through the load connection terminals. In many cases, the controller is provided with overload protection means to stop the output of the inverter circuit when an overload current flows through the inverter circuit and the filter circuits in order to protect them.
The DC power supply section comprises a rectifier to rectify the output of the AC generator and a smoothing capacitor connected across the DC output terminals of the rectifier to generate a DC voltage across the smoothing capacitor.
The generally used inverter circuit comprises a bridge type switch circuit having a plural of switch arms connected in parallel to each other and each having an upper arm switch element and a lower arm switch element connected in series to each other. A pair of DC input terminals are led out of a common connection point of both ends of the switch arms while AC output terminals are led out of the connection point of the corresponding switch elements of each of the switch arms.
This inverter circuit has feedback diodes provided corresponding to the switch elements and connected in reverse parallel to the corresponding switch elements, respectively. Thus, A diode bridge full wave rectifier circuit is formed by the feedback diodes as viewed on the AC output terminals of the inverter circuit.
The PWM control means of the controller serves to output from the inverter circuit the AC voltage of intermittent waveform having a duty value xe2x80x9cDxe2x80x9d changing for every PWM cycle in accordance with an instant value of the AC output voltage applied through the load connection terminals to a load by carrying out the on-off operation of the pair of switch elements located at the diagonal position of the bridge of the inverter circuit at predetermined timing with a PWM signal of, pulse waveform applied to the pair of the switch elements.
The duty value xe2x80x9cDxe2x80x9d of the PWM control can be determined by multiplying a correction coefficient Kv by a reference duty value xe2x80x9cDoxe2x80x9d required for making a waveform of an AC output voltage to be output from the load connection terminals into predetermined waveform. In general, a ratio VA/VD of a rated value VA of the AC output voltage obtained across the load connection terminals to a DC power supply voltage VD is used for the correction coefficient Kv.
The AC voltage output from the inverter circuit is converted into an AC output voltage of smooth waveform having the harmonic component removed by the filter circuit.
The overload protection means provided in the controller controls the inverter circuit to stop the operation of the inverter when the load current equal to or more than the overload judgment current value flows through the load connection terminals whereby the switch elements of the inverter circuit is protected from the overload current.
With the inverter generator apparatus, since the DC voltage output from the DC power supply section is converted into the AC voltage of arbitrary frequency by controlling the inverter circuit, the AC voltage of predetermined frequency can be obtained from the load connection terminals in spite of what the revolution of the generator is. Also, the AC voltage of arbitrary magnitude can be obtained by controlling the duty value changing for every PWM cycle in the PWM control.
In case that the volume of the load is large, the aforementioned inverter power generation apparatus is sometimes required to be operated in parallel to the other AC power supply apparatus (referred to as an external AC power supply later). What is meant by xe2x80x9cthe external AC power supplyxe2x80x9d may be another inverter power generation apparatus formed in the same manner, an AC generator to generate an output voltage of frequency identical to that of the inverter power generation apparatus or a commercial AC power supply.
When the inverter power generation apparatus and the external AC power supply are operated in parallel to each other, the load connection terminals of both the inverter power generation apparatus and the external AC power supply are connected to each other and the electric power is simultaneously supplied from the inverter power generation apparatus and the external AC power supply to the load. In this case, the voltages generated by the inverter power generation apparatus and the external AC power supply are required to have a waveform, a crest value and a phase identical to each other.
Thus, in case that the inverter power generation apparatus and the external AC power supply are operated in parallel to each other, when the AC voltage is applied between the load connection terminals from the external power supply, the controller should control the inverter power generation apparatus so that the waveform, the crest value and the phase of the output voltage of the inverter power generation apparatus are coincident with those of the voltage applied from the external power supply
In the specification, it is premised that the external power supply having the output frequency equal to the output frequency of the inverter power generation apparatus is selected when they are operated in parallel to each other. In other words, it is premised that the output frequencies of the inverter power generation apparatus and the external AC power supply are coincident with each other.
The applicant has proposed an inverter power generation apparatus adapted to control an inverter circuit so that a waveform, a crest value and a phase of an AC output voltage applied across load connection terminals from the inverter circuit are coincident with those of an AC voltage applied from the external AC power supply from which the AC voltage is applied across the load connection terminals, which is disclosed in JP11-182294 and JP11-284277.
In the invention of JP11-182294, when an overcurrent flows through the inverter power generation apparatus, the output of the inverter power generation apparatus is once stopped and it is judged whether the overcurrent is caused by the overload or by the external AC power supply connected across the load connection terminals. When it is judged that the overcurrent is caused by the overload, the output of the inverter power generation apparatus is kept stopped and when it is judged that the overcurrent is caused by the voltage applied from the external AC power supply across the load connection terminals, the output of the inverter power generation apparatus is restored after the waveform, the crest value and the phase of the AC output voltage applied across the load connection terminals from the inverter circuit are made coincident with those of the AC voltage applied from the external AC power supply.
In the inverter power generation apparatus shown in JP11-284277, in case that the inverter circuit is controlled so as to output an AC output voltage of reference waveform from the inverter circuit through the load connection terminals, the difference between the phases of the reference waveform and the waveform of the actual output across the load connection terminals is detected and when the phase difference is beyond the set range, the phase of the AC voltage applied from the inverter circuit across the load connection terminals is so controlled as to be coincident with that of the AC voltage applied from the external power supply by shifting the phase of the reference waveform.
With the aforementioned control performed, in almost all cases, the parallel operation of the inverter power generation apparatus and the external power supply can be accomplished without any trouble. However, the aforementioned control is performed so that the waveform, the crest value and the phase of the output voltage of the inverter power generation apparatus are corrected in accordance with the information of the voltage applied from the external power supply across the load connection terminals, but not performed so that the waveform, the crest value and the phase of the output voltage of the inverter power generation apparatus are always coincident with those of the voltage applied from the external power supply by synchronizing the inverter power generation apparatus and the external AC power supply. Thus, it is hard that the balance between the output voltage of the inverter power generation apparatus and the voltage applied from the external AC power supply is fully prevented from being lost.
In the condition where the inverter power generation apparatus and the external AC power supply are operated in parallel to each other, when the crest values and the phases of the output voltage of the inverter power generation apparatus and those of the output voltage of the external AC power supply are shifted by any cause so that the balance between the outputs of the inverter power generation apparatus and the external AC power supply collapses. In some cases, the current is caused to flow from the side of the external AC power supply via the load connection terminals through the inverter power generation apparatus due to the collapse of the balance. This causes the inverter power generation apparatus to serve as a load to the external AC power supply and therefore since the electric power cannot be supplied from the inverter power generation apparatus to the load, the object of the parallel operation in which the electric power is supplied from the two power supplies cannot be accomplished.
As the current flows from the external AC power supply through the inverter power generation apparatus, the smoothing capacitor of the DC power supply section is charged through the full-wave rectifier circuit formed by the feedback diodes provided in the inverter circuit and therefore the output voltage of the DC power supply section increases. At that time, since the correction coefficient Kv for determining the duty value decreases, the duty value for the PWM control decreases so that the output voltage of the inverter power generation apparatus decreases. Thus, since the current flowing from the side of the external AC power supply through the inverter circuit further increases. This enhances the unbalance between the output of the inverter power generation apparatus and the output of the external AC power supply and therefore the parallel operation of the inverter power generation apparatus and the external AC power supply gets fully meaningless. Also, since the inverter power generation apparatus serves as the load to the external AC power supply and the latter gets the state of overload, in case that there is provided means to limit the load current in the external AC power supply, the output of the external AC power supply is sometimes interrupted so that the electrical conduction to the load is stopped.
Accordingly, it is a principal object of the invention to provide a method of controlling an inverter power generation apparatus adapted to being operated in parallel with an external AC power supply without any trouble so that any current is prevented from flowing from the external AC power supply through the inverter power generation apparatus.
The invention is applied to a method of controlling a parallel operation of two inverter power generation apparatuses each comprising an AC generator, a DC power supply section having a rectifier to rectify an output of the AC generator and a smoothing capacitor connected across DC output terminals of the rectifier circuit, an inverter circuit having on-off controllable switch elements to convert a DC output voltage of the DC power supply section into an AC voltage by switching an operation of the switch elements, a filter circuit to remove a harmonic component from an output from the inverter circuit, load connection terminals having the output of the filter circuit applied whereby the inverter circuit is so controlled as to output the AC voltage of predetermined waveform through the load connection terminals when no voltage is applied from the external AC power supply across the load connection terminals and when the voltage is applied from the external AC power supply across the load connection terminals, a waveform, a crest value and a phase of the AC voltage output from the filter circuit are coincident with those of the AC voltage applied from the external AC power supply, respectively.
The method of the invention controls the two inverter power generation apparatuses by detecting the DC power supply voltage of each of the two inverter power generation apparatuses, limiting a load current of each of the inverter power generation apparatuses to a value equal to or less than the maximum rated load current allowed for outputting the AC voltage of waveform having no distortion from each of the inverter power generation apparatuses when both of the detected DC power supply voltages of the two inverter power generation apparatuses are equal to or less than a judgment value set at a value equal to or more than a non-load output voltage of the DC power supply section of the corresponding generation apparatuses and allowing the load current of one of the inverter power generation apparatuses to exceed the value of the maximum rated load current when the DC power supply voltage of the other inverter power generation apparatuses exceeds the judgment value.
In general, the AC generator has a characteristic in which the output voltage gets lowered as the output current increases. Thus, the DC power supply voltage of the inverter power generation apparatus increases as the load current decreases and gets approximately equal to the crest value of the no-load output voltage of the AC generator when non-loaded. In the normal state where no current flows from the side of the load connection terminals of the inverter power generation apparatus to the side of the DC power supply section, the DC power supply voltage never exceeds the no-load output voltage of the AC generator.
On the other hand, as the current flows from one of the inverter power generation apparatuses to the other inverter power generation apparatus when the balance between the output voltages of the two inverter power generation apparatuses collapses due to any cause, the smoothing capacitor of the former inverter power generation apparatus is charged by the current. Thus, the output voltage (the DC power supply voltage) of the DC power supply section is higher than the no-load output voltage of the DC power supply section.
Accordingly, with the DC power supply voltage of each of the inverter power generation apparatuses detected and whether the DC power supply voltage is equal to or less than the judgment value set at the value equal to or more than the no-load output voltage of the DC power supply section or not judged as aforementioned, whether there collapses the balance between the output of the inverter power generation apparatus and the output of the external AC power supply or not (whether the current flows to one of the inverter power generation apparatuses from the other inverter power generation apparatus or not) can be judged.
In this manner, when it is judged that the current flows to one of the inverter power generation apparatuses from the other inverter power generation apparatus, the load current of the other inverter power generation apparatus is so controlled as to be allowed to exceed the value of the maximum rated load current, which causes the output voltage of the other inverter power generation apparatus to be lowered by the overload current. Thus, the current no longer flows to one of the inverter power generation apparatuses from the other inverter power generation apparatus and therefore, the DC power supply voltage of the one inverter power generation apparatus decreases. Thus, the load current flows from the one inverter power generation apparatus as well and as a result, the parallel operation of the two inverter power generation apparatuses can be restored.
When both of the load current of the two inverter power generation apparatuses get excessive, the outputs of these inverter power generation apparatuses are stopped and therefore the overcurrent can be prevented from flowing.
In a preferred-embodiment, there is provided a controller to perform.a PWM control in which a PWM signal is applied to the control terminals of the switch elements of the inverter circuit so as to output from the inverter circuit the AC voltage of intermittent waveform having the duty value varying for every PWM cycle in accordance with an instant value of the AC output voltage applied to the load through the load connection terminals and an overload protection control in which the output of the inverter circuit is stopped when the value of the DC power supply voltage is lower than the overload judgment voltage value.
In case that the inverter power generation apparatus the invention is applied to is provided with the aforementioned controller, it is so constructed as to control so that the waveform, the crest value and the phase of the AC output voltage applied across the load connection terminals from the inverter circuit when the external AC voltage is applied from the external AC power supply to the load connection terminals is made coincident with those of the external AC power supply voltage and performs the power supply voltage judgment step in which whether the detected DC power supply voltage of each of the inverter power generation apparatuses is equal to or less than the judgment value or not is judged.
When it is judged in the power supply voltage judgment step that the DC power supply voltage is equal to or less than the judgment value, with the value of the DC power supply voltage when the maximum rated load current allowed for outputting the AC voltage having no waveform distortion through the load connection terminals of each of the inverter power generation apparatus flows through the inverter circuit of each of the inverter power generation apparatus to the side of the load determined as the value of the minimum power supply voltage in the steady state, the value of the overload judgment voltage for the overload protection control of each of the inverter power generation apparatuses is set at a value equal to the value of the steady state minimum power supply voltage so that the load current of each of the inverter power generation apparatuses is prohibited from exceeding the value of the maximum rated load current. When it is judged in the power supply voltage judgment step that the DC power supply voltage of one of the inverter power generation apparatuses exceeds the judgment value, the value of the overload judgment voltage for the overload protection control of the other inverter power generation apparatus is set at a value further lower than the value of the steady state minimum power supply voltage so that the overload operation of the other inverter power generation apparatus is allowed.
Although, in the aforementioned embodiment, the overload state is determined by detecting the DC power supply voltage, the overload protection control can be performed so that the output of the inverter circuit is stopped when it is judged that the load current exceeding the value of the overload judgment current flows from the inverter circuit by detecting the load current of each of the inverter power generation apparatuses.
In case that the inverter power generation apparatus the invention is applied to is controlled as aforementioned, when it is judged in the power supply voltage judgment step that the DC power supply voltages of the two inverter power generation apparatuses are equal to or less than the judgment value set at the value equal to or more than the no-load output voltage of the corresponding DC power supply section, a value of a overload judgment current of each of the inverter power generation apparatuses is set at the value equal to the value of the steady state maximum rated load current allowed for outputting the AC output voltage having no waveform distortion from the corresponding inverter power generation apparatus so that the load current of each of the inverter power generation apparatuses is prohibited from exceeding the value of the maximum rated load current. When it is judged in the power supply voltage judgment step that the DC power supply voltage of either of the inverter power generation apparatuses exceeds the judgment value, the value of the overload judgment current of the other inverter power generation apparatus is set at the value larger than the value of the maximum rated load current so that the overload operation of the other inverter power generation apparatus in which the load current thereof exceeds the maximum rated load current is allowed.
In the inverter power generation apparatus proposed by the applicant, the PWM control makes as the duty value xe2x80x9cDxe2x80x9d the value obtained by multiplying by the correction coefficient Kv the reference duty value xe2x80x9cDoxe2x80x9d required for obtaining the predetermined waveform of the AC output voltage output from the load connection terminals and the controller is so formed that the waveform, the crest value and the phase of the AC output voltage applied across the load connection terminals from the inverter circuit when the external AC voltage is applied from the external AC power supply across the load connection terminals are made coincident with those of the external AC voltage.
In case that the invention is applied to the inverter power generation apparatus having the controller aforementioned, there are used AC generators having the same characteristic for the two inverter power generation apparatuses and it is judged whether the detected DC power supply voltages of the two inverter power generation apparatuses are equal to or less than the judgment value set at the value equal to or more than the no-load output voltage of the DC power supply section of the corresponding inverter power generation apparatuses or not. It is judged in the power supply judgment step that both of the DC power supply voltages of the two inverter power generation apparatuses are equal to or less than the corresponding judgment value, ratios of the rated value VA of the crest value of the AC voltage output through the load connection terminals relative to the respective DC power supply voltages VD and VDxe2x80x2 of the two inverter power generation apparatuses which can be expressed by VA/VD and VA/VDxe2x80x2, respectively are used as the correction coefficient Kv for the PWM control of the two inverter power generation apparatuses, respectively. When it is judged in the power supply voltage judgment step that the DC power supply voltage VD of one of the inverter power generation apparatuses exceeds the judgment value, the ratio VA/VDxe2x80x2 of the rated value VA of the crest value of the voltage across the load connection terminals relative to the DC power supply voltage VDxe2x80x2 of the other inverter power generation apparatus is used as the correction coefficient Kv for the PWM control of the one inverter power generation apparatus.
In this manner, since when it is judged that the DC power supply voltage of one of the inverter power generation apparatuses exceeds the judgment value equal to or more than the crest value of the no-load output voltage of the AC generator; that is, when the load current of the other inverter power generation apparatus flows through the one inverter power generation apparatus, for the correction coefficient Kv by which the reference duty value is multiplied for determining the duty value for the PWM control of the one inverter power generation apparatus (the inverter power generation apparatus to which the current flows from the side of the load connection terminals to the side of the DC power supply section) is used the ratio VA/VDxe2x80x2 of the rated value of the crest value of the AC output voltage applied to the load through the load connection terminals relative to the DC power supply voltage VDxe2x80x2 of the other inverter power generation apparatus, the duty values of the PWM signals applied to the inverter circuits of the two inverter power generation apparatuses can get equal to each other and therefore the unbalance between the outputs of the two inverter power generation apparatuses can be cancelled so that the current can be prevented from flowing to the one inverter power generation apparatus from the other inverter power generation apparatus.
The invention may perform the simultaneous PWM control of the two inverter power generation apparatuses by applying a timing signal for determining a start timing of every PWM cycle of the PWM control made for one of the two inverter power generation apparatuses to the other inverter power generation apparatus and determining the start timing of every PWM cycle of the PWM control made for the other inverter power generation apparatus on the timing signal applied from the one inverter power generation apparatus. Since the outputs of the two inverter power generation apparatuses can be always balanced by performing this control, the state where the current flows from the one inverter power generation apparatus to the other inverter power generation apparatus due to the collapse of the balance between the outputs of the two inverter power generation apparatuses can be prevented and the parallel operation of the two inverter power generation apparatuses can be performed without any trouble.
In case that this control is made, the overload state may be judged for the overload protection control either by detecting the output voltage of the DC power supply section or by detecting the load current.
Also, in the invention, when it is judged in the power supply judgment step that the DC power supply voltage is equal to or less than a judgment value set at a value equal to or more than the crest value of the no-load output voltage of the AC generator, for the correction coefficient Kv may be used the ratio VA/VD of the rated value VA of the crest value of the voltage output through the load connection terminals relative to the DC power supply voltage VD of each of the inverter power generation apparatuses and when it is judged in the power supply judgment step that the DC power supply voltage exceeds the judgment value set at the value equal to or more than the crest value of the no-load output voltage of the AC generator, for the correction coefficient Kv may be used the ratio VA/VDo of the rated value VA relative to a provisional DC power supply voltage VDo set at a value equal to or less than the no-load voltage Vo of the AC generator.
As the ratio VA/VDo of the rated value VA relative to the provisional DC power supply voltage VDo set at the value equal to or less than the no-load voltage Vo of the AC generator is used for the correction coefficient Kv for arithmetically operating the duty value of the PWM signal when the DC power supply voltage of the inverter power generation apparatus exceeds the judgment value set at the value equal to or more than the crest value of the no-load voltage because the state where the current flows from the external AC power supply through the load connection terminals into the side of the inverter circuit occurs due to any cause, the state where the load current of the inverter power generation apparatus is made zero or some current flows arises whereby the current can be avoided from flowing through the inverter power generation apparatus in a reverse direction. At that time, since the load current of the external AC power supply operated in parallel to the inverter power generation apparatus is fully consumed by the load, the load current of the external AC power supply increases and the output voltage thereof is lowered. As the output of the external AC power supply is lowered, the DC power supply voltage of the inverter power generation apparatus is lowered and therefore the normal state where the current is supplied from both of the inverter power generation apparatus and the external AC power supply is restored. In this case, the external AC power supply to be operated in parallel to the inverter power generation apparatus may be the inverter power generation apparatus having the same construction, or may be other power generation apparatus or a commercial power supply.
In this invention, when it is judged in the power supply voltage judgment step that the DC power supply voltage exceeds the judgment value, the phase of the PWM signal applied to the control terminals of the switch elements of the inverter circuit may be shifted until the DC power supply voltage is equal to or less than the judgment value.
With such a control performed, when the balance between the output of the inverter power generation apparatus and the output of the external AC power supply voltage collapses due to any cause, the state where the balance can be restored or the waveform, the crest value and the phase of the output voltage of the inverter power generation apparatus are coincident with those of the AC voltage applied from the external AC power supply can be restored. Thus, the state where the current flows from the side of the external AC power supply through the load connection terminals into the side of the inverter circuit can be avoided.
In this case, the external AC power supply may be the inverter power generation apparatus of the same construction or other power generation apparatus or the commercial power supply as well.